The intent of hydraulic fracturing used for well stimulation is to increase well productivity by creating a highly conductive path (relative to formation permeability) through which hydrocarbons may flow. The fracture is initiated by pumping a suitable fluid, with or without proppant suspended therein, into the formation at a sufficient rate to overcome the earth's compressive forces.
As injection of a fracturing fluid containing a propping agent continues, the proppant is placed within the fractured zone. After completion of the fracturing treatment, the fluid is recovered from the formation and the proppant remains to hold the fracture faces apart.
A fracturing fluid is pumped into a well bore through a conduit like tubing or casing which may range from 27/8 to 7 inches in diameter. The pumping equipment and tubular goods are limited by specific safety and pressure requirements which can be prohibitive. If the wellhead pressure and hydraulic horsepower required to overcome both the earth's compressive forces and friction loss exceed pump capacity and/or tubular conduit strength, a fracturing fluid of desirably high viscosity cannot be injected at a sufficient rate to effectively open a fracture. It is therefore desirable that the viscosity of the fluid be initially low enough that excessive friction losses and high wellhead pumping pressures are not encountered, yet ultimately high enough to produce a fracture in the formation.
In conventional hydraulic fracturing applications where high viscosity is desired, crosslinking compounds are mixed with aqueous solutions containing hydratable polysaccharides such as galactomannan gums, glucomannan gums, and cellulose derivatives. Most appropriate cross-linking compounds provide metal ions which crosslink polymer strands through available ionic sites. Cross-linking can occur through hydroxy, amino, amido, carboxyl, and thio groups although the hydroxy crosslinking reaction occurs most readily. The crosslinking reaction of a common guar derivative such as hydroxypropyl guar with a frequently used crosslinking agent like triethanolamine titanate generally occurs within seconds. This reaction increases viscosity by several orders of magnitude.
Two potentially undesirable characteristics may be correlated with the immediate increase in viscosity associated with conventional crosslinked systems. As viscosity increases, frictional pressure increases; thus, rapid viscosity development dramatically affects operating pressure. Gels are subject to shear as they pass through pumping equipment and tubular goods. Introducing shear to a completely crosslinked gel, i.e. a gel which has attained full viscosity, can result in a disruption of the crosslinked structure structure which can reduce the ultimate stability of the crosslinked gel.
Conway U.S. Pat. Nos. 4,502,967 and 4,470,915 teach a means for delaying crosslinking reactions by admixing predetermined quantities of retarding agent and crosslinking agent such that a retarded crosslinking composition is formed which in itself provides a delayed crosslinking reaction. This method requires production of retarded crosslinking compositions having preselected properties controlled by the volumetric ratios of crosslinking compound to retarding agent, and the time and temperature of the aging period following blending. Acceptable retarding agents for such applications include polyhydroxycarboxylic acids having 3 to 7 carbon atoms and polyhydroxyl-containing compounds. Polyhydroxyl-containing compounds may also be batch mixed in predetermined amounts with the base gel prior to the addition of a crosslinking agent, Hollenbeak U.S. Pat. No. 4,464,270. The base gel containing the retarding agent is introduced into the wellbore in admixture with a crosslinking agent. Crosslinking agents for use with gels containing batch mixed retarding agents or crosslinker/retarder blends are organotitanates.
Batch-mixing crosslinking rate controllers or premixing crosslinker/retarder blends provide minimal flexibility. Batch-mix processes and premixed blends generally necessitate estimating a desire crosslinking reaction time prior to pumping. In batch-mixing procedures, the quantity of rate retarder may vary according to the common fluid variables. Consequently, the required amount of the crosslinking rate controller must be determined for each individual tank of fracturing fluid and the addition of that amount must be meticulously controlled. After a rate controller has been added, any unforeseen changes in fluid conditions such as changes in surface temperature, viscosity or pH can cause corresponding variations in crosslinking reaction times.
Crosslinker/rate controller blends must also be mixed before injection of the fracturing fluid. A desired crosslinking time is chosen and either one crosslinker/rate controller blend is prepared which provides an average crosslinking reaction time over a range of fluid variables or several blends are prepared to compensate for changes in fluid composition. Again unforeseen changes in fluid conditions can cause changes in crosslinking times for which compensation is difficult after crosslinker/rate controller blends have been prepared.
Due to the difficulties in the application of batch-mixed processes and crosslinker/rate controller blends, it is preferable in the practice of the present invention to deliver the crosslinking agent and crosslinking rate controller simultaneously while pumping the fracturing fluid into the wellbore. By simultaneously injecting the two components, the crosslinking reaction rate can be monitored throughout the fracturing operation and the quantity of rate controller may be altered if necessary to compensate for changing conditions.
The method of the present invention employs aldehydes and dialdehydes as retarding agents and organozirconates as crosslinking agents. The invention further extends the users' ability to control crosslinking time by providing a crosslinking rate accelerator comprising polyamines. Crosslinking rate retarders or accelerators in the preferred practice of the invention are injected into the gel simultaneously with, yet independently of, the crosslinking agent while pumping the fluid into the formation.